B200s hum

[duecemfhatchet]

I have a B200s power amp that has a extremely loud buzz or hum. It has steadily gotten louder to the point of almost being unusable. Any ideas where to start?

[MarshallJMP]

I would start with the big caps in the power supply. It could also be that one of the transistor is drawing to much current. Do you have DC voltage on the output?
Do you have hum on both channels?

[duecemfhatchet]

Yes it is in both channels and no DC voltage at outputs.

[MarshallJMP]

In that case I would suspect a faulty filter cap.

[rnolan]

Hey Duece, did you get this sorted?

[Harley Hexxe]

I know one of my B200S amps buzzes like a hornet's nest, and I'm sure it's the filter caps.

I just hope they're still available at Mouser.

[rnolan]

Hey Harley, maybe it's the caps , but I've had a couple of B200s, and the caps are still fine.  First up, reflow (if you haven't already) all the solder joins, I did the whole amp when I did it as it doesn't take long.  I was fixing bad pops and crackles, MJMP said reflow all the joints around the power supply, now all fixed .  So maybe try that first.

Also consider, the jacks (in and out) need to be isolated from the chassis (they have plastic sheaves).  If you replace a speaker jack (with standard jack) and have it grounding to the chassis, it will hum badly. As my friend MikeB found out...

[Harley Hexxe]

Hey Richard,

  Thanks for the tip on the speaker jacks, I didn't know about that one. I was thinking about replacing them on one of my B200Ss. They were missing the plastic nuts when I got it. This amp was well used before I bought it, and it's also the one that buzzes. I did spot a bulged dome on one of the large filter caps, so it's time for a new set, there's no mistake about that.

  I will reflow the solder on the other one I have after checking under the hood and hit all the pots with deoxit. 

[MarshallJMP]

You can buy the nuts separate, even in different colors.

[rnolan]

IIRC you can also get plastic insulating sheaths to prevent the socket connecting to the chassis, that's how MikeB fixed his. Or is that how the nuts come MJMP? maybe you just need those .  From memory though, I think Mike bought some sheaths and could use the metal washers.

Upside of the re-flow, there aren't that many solder joins in a B200s, it didn't take me long to do them all and it was like new after .  But as you say, you need some power caps.

[Harley Hexxe]

You can buy the nuts separate, even in different colors.

  It's been a few years since I looked at it, but if I remember correctly, one of the threaded sleeves is half gone. It was that way when I bought it, so I may have to shop for some replacement jacks too.

  I'm not in a big hurry though, I want to go through all my remaining MT100s, and replace the output transistors and maybe recap them at the same time. After I have those up and running, I'll focus on the B200S amps.

[duecemfhatchet]

Never figured it out. Still noisy buzz. I did replace the big caps and nothing different. Since then I left it alone while it was still working. I don't have a lot of confidence with my repair skills lately.

[Harley Hexxe]

Are the schematics for the B200S in the vault?

We should have it in there. My guess at this time would be to check the values of the resistors connected to the filter caps to see if they've drifted out of tolerance. After 35+ years, it's very likely.

  When I get to my B200S, I'll be checking those out too as well as recapping elsewhere on the board. (I would use carbon film resistors in the power filtering section).

[rnolan]

Hey Duce, if you haven't done this already (and you have replaced the filter caps), try re-flowing all the solder joins.  The bottom comes off and makes this reasonably easy.  It fixed my B200s.  Also make sure the jacks don't ground to the chassis, although that tends to make a horrible hum not so much a buzz.

That's not to say there isn't a different problem (that MJMP can help with more than me) but it's a good starting point.

When I did the re-flow, some of the joins needed some additional solder, some didn't.  Obviously be careful not to over heat the join and lift a pad .

[rnolan]

Hey Harley, yes all the B200s schematics we have are posted in the vault